$\begin{figure} \par\includegraphics[width=8.8cm,clip]{Eh151_f3.eps}\end{figure}$

Figure 3: Fraction of CO molecules in the solid phase at different temperatures as function of density. Adsorption onto H₂O and CO ice produce different results. The desorption energies are from Sandford & Allamandola (1988) for CO-H₂O and Sandford et al. (1988) for CO-CO.

A simple disk model cf. Chiang & Goldreich (1999) with T_*=3500 K and a disk radius of 250 AU has been adopted to investigate the location of the solid and gaseous CO seen in infrared absorption. Because of its higher temperature, the gaseous CO is likely not co-located with the bulk of the apolar solid CO, which evaporates at $\sim$ 20 K. The polar solid CO can however reside in the same region of the disk as the CO gas at 40-60 K. Assuming that CO is frozen out at <20 K (apolar CO) and <40 K (polar CO), the best fit to the column densities, gas/solid CO and polar ice/apolar ice ratio is obtained for $i=20\pm 5\hbox{$^\circ$ }$ . This inclination is consistent with the flux asymmetry seen in the near-infrared image of Brandner et al. (2000). For such line of sight, most of the CO ice is located above the midplane in the outer disk, whereas the CO gas is found in the warm inner disk. Thus, the overall CO depletion could be significantly higher than the ratio of $\sim$ 1 found here.

Several time-dependent chemical models were run to quantify the gas/solid CO ratios in different density and temperature regimes. The models simulate gas-phase chemistry, freeze-out onto grain surfaces and thermal as well as non-thermal evaporation. Cosmic-ray induced desorption is modeled as in Hasegawa & Herbst (1993), which may be an overestimate for large grains in disks (Shen et al., in prep.). The sticking coefficient was set at 0.3 to account for other non-thermal mechanisms (e.g., Schutte & Greenberg 1997) and photodesorption is assumed ineffective. At $T>T_{\rm evap}$ thermal desorption dominates, whereas at $T<T_{\rm evap}$ cosmic-ray desorption prevails. In the model, $T_{\rm evap}$ is 20 K for apolar CO ice ( $E_{\rm des}=960$ K) and 40 K ( $E_{\rm des}=1740$ K) for polar CO. To illustrate the effects of thermal and cosmic-ray induced desorptions, Fig. 3 shows the gas/solid CO ratio at chemical equilibrium. In cold (T< 20 K) but moderately dense (10⁴-10⁵ cm^-3) regions of the disk around CRBR 2422.8-3423, only a small amount of CO is depleted onto grains in the apolar form, but much larger fractions >50% can occur at higher densities ( $\sim$ 10⁶-10⁸ cm^-3).

In summary, we detected a large amount of solid CO in the line of sight toward CRBR 2422.8-3423. The majority of this ice is likely located in the flaring outer regions of the edge-on circumstellar disk. Very high resolution (R>10⁵) near-infrared spectroscopy is needed to reveal the gaseous CO line profiles and thus their origin and the gas dynamics in the inner disk. Future submillimeter interferometer data can probe the velocity pattern and excitation conditions of the gas as functions of disk radius, whereas mid-infrared spectroscopy with, e.g., the Space Infrared Telescope Facility (SIRTF) will allow searches for other ice components, in particular solid CO₂.

5 Discussion